Optical pickup device

ABSTRACT

An optical pickup device which includes (A) a light source, and (B) an objective lens and an aberration correcting optical system which are located in a light path from the light source to an optical recording medium is so arranged that the aberration correcting optical system imparts a phase distribution to luminous flux which transmits the aberration correcting optical system, so as to correct a predetermined aberration; and an amount of phase of the aberration correcting optical system when correcting the aberration is set in such a manner that the aberration correcting optical system imparts a larger amount of phase at a position farther from a point where the aberration correcting optical system crosses an optical axis of light emitted from the light source. With this, it is possible to increase a tolerance for the center misalignment of the objective lens so as to reduce aberration caused by the center misalignment.

TECHNICAL FIELD

[0001] The present invention relates to optical pickup devices forrecording and reproducing information on and from an optical informationrecording medium.

BACKGROUND ART

[0002] Optical technology has many characteristics such as capability ofhigh-speed processing at a frequency of light (speed is high because ofhigh frequency), capability of spatial information processing, andcapability of phase processing. For this reason, the optical technologyhas been researched, developed, and put into practical use in a widerange of fields such as communications, measurement, and processing.

[0003] Such optical technology uses a high-precision objective lens tofocus a light beam.

[0004] In accordance with a recent growing demand particularly for imagerecording devices utilizing light, technique aiming to increase thestorage capacity of an optical information recording medium has becomequite important. In order to increase the storage capacity of an opticalinformation recording medium, the quality of the recording medium needsto be improved, and what is more, a beam spot needs to have a smallerdiameter, namely the objective lens needs to sufficiently focus the beamspot.

[0005] As is well known, the diameter of the beam spot is proportionalto the wavelength of light, and is inversely proportional to the NA(Numerical Aperture) of the objective lens. In other words, it isnecessary to either shorten the wavelength of light or increase the NAof the objective lens, in order to focus the beam spot having a smallerdiameter.

[0006] In order to shorten the wavelength of light, a blue laser diodeand a blue or green SHG laser have been recently developed. On the otherhand, in order to increase the NA of the objective lens, a higherdensity has been achieved in DVD (Digital Versatile Discs) whose NA is0.6 compared with CD (Compact Disc) whose NA is 0.45. Further, JapaneseUnexamined Patent Publication No. 123410/1998 (Tokukaihei 10-123410,published on May 15, 1998) discloses an optical pickup device aiming tohave an increased density. This optical pickup device uses a 2-grouplens in which two lenses are combined, so that the objective lens has anNA of 0.85.

[0007] The optical pickup device which uses the objective lens having ahigh NA as described above is required to correct (i) unevenness inthickness of a light transmission layer of an optical recording mediumand (ii) a spherical aberration which occurs in multi-layer recording.For example, Japanese Unexamined Patent Publication No. 143303/2001(Tokukai 2001-143303, published on May 25, 2001) discloses a liquidcrystal element which corrects the spherical aberration.

[0008] The liquid crystal element of the foregoing Publication isarranged such that liquid crystal is sandwiched between electrodes whichare respectively formed on glass substrates. In order to correct thespherical aberration, the liquid crystal element applies a voltage tothe electrodes so as to change the alignment of the liquid crystal. Thischanges a refractive index of the liquid crystal, thereby forming aphase distribution.

[0009] As an example, the following describes a system in which theobjective lens has an NA of 0.85 and the light transmission layer of theoptical recording medium has a thickness of 0.1 mm. In the event where alight transmission layer with a thickness of 0.115 mm is used, a phasedistribution as shown in FIG. 8 is imparted to the liquid crystalelement so as to correct the spherical aberration caused by theincreased thickness. This phase distribution is for a sphericalaberration corresponding to the increased thickness (+15 μm) of thelight transmission layer, and it is required to correct a sphericalaberration remaining on a best image plane after focusing. In otherwords, the phase distribution is formed to reduce spherical aberration,and it is required to correct a spherical aberration that remains afterthe objective lens was moved in a direction of the optical axis in thefocus operation that is carried out to match a recording-reproducingsurface with the best image plane on which a wavefront aberration is thesmallest.

[0010]FIG. 9 shows a relationship between radius r of the liquid crystalelement and the second derivative of the phase distribution, wherein thesecond derivative indicates the rate of change (rate of fluctuation) inthe slope of the phase. As shown in FIG. 9, the phase distribution hastwo inflection points. Further, the graph of FIG. 9 has considerablylarge values at outermost portions of the effective light beam. Here,imparting such a phase distribution to the liquid crystal element doesnot cause a problem when the central axis of the liquid crystal elementis aligned with the central axis of the objective lens. However, whenthe central axis of the liquid crystal element and the central axis ofthe objective lens are misaligned with each other even slightly in theradius direction of the liquid crystal element and the objective lens,the wavefront of incident light on the objective lens is greatlydisturbed, thereby causing a large aberration.

[0011] Therefore, the foregoing arrangement has a problem that thecharacteristics largely deteriorate due to the center misalignment ofthe liquid crystal element with the objective lens. Further, the liquidcrystal element and the objective lens need to be precisely aligned witheach other in order to prevent deterioration of the characteristics,thereby forcing complicated work on a user. Further, when the objectivelens mounted on an actuator is moved in a direction perpendicular to theoptical axis and perpendicular to the tracking direction in order tocarry out a tracking operation, the center misalignment occurs such thatthe optical axis of the light beam entering the objective lens ismisaligned with the central axis of the objective lens. Because suchcenter misalignment is not allowable, it is also necessary to mount boththe objective lens and the liquid crystal element together on theactuator.

[0012] Further, in order to allow linearly polarized light to enter theliquid crystal element, it is necessary to provide a quarter-wave plateat a position closer to the optical recording medium than the liquidcrystal element, namely on the actuator. This increases the weight ofparts mounted on the actuator (weight of moving parts), and thusprevents high-speed driving, making it difficult to increase the speedof recording and reproducing.

[0013] Another problem of mounting the liquid crystal element on themoving part (actuator) is that it becomes difficult to route lead wiresfor applying a voltage to the liquid crystal element, or to position aflexible substrate.

[0014] In view of the foregoing problems, the present invention has anobjective to provide an optical pickup device for reducing theaberration caused by position misalignment of the liquid crystal elementwith the objective lens, even if the objective lens has a high NA.

DISCLOSURE OF INVENTION

[0015] In order to attain the foregoing object, an optical pickup deviceof the present invention which includes a light source; and an objectivelens and an aberration correcting optical system which are located in alight path from the light source to an optical recording medium is soarranged that the aberration correcting optical system has a phasedistribution for a light beam passing through the aberration correctingoptical system, so as to correct a predetermined aberration; and theaberration correcting optical system for correcting an aberration is setso that a magnitude of a phase in the phase distribution increases withincrease in distance from a point where the aberration correctingoptical system crosses an optical axis of light emitted from the lightsource.

[0016] With this arrangement, even if the center of the objective lensis not aligned with the center of the aberration correcting opticalsystem, the rate of change in the slope of the phase is small because amagnitude of a phase monotonously changes. Therefore, the centermisalignment does not cause a large aberration if the objective lens andthe aberration correcting optical system have been appropriatelyadjusted in a state having no center misalignment.

[0017] This eliminates the need for mounting both the objective lens andthe aberration correcting optical system together on an actuator, forexample.

[0018] The weight of the actuator can thus be reduced, thereby providingan optical pickup device capable of driving the actuator at high speed.

[0019] The optical pickup device of the present invention may bearranged so that the aberration correcting optical system for correctingan aberration is set so that a magnitude of a phase in the phasedistribution increases with increase in distance from a point where theaberration correcting optical system crosses an optical axis of lightemitted from the light source within an effective diameter of theaberration correcting optical system.

[0020] Further, in order to attain the foregoing object, the opticalpickup device of the present invention is arranged so that the phasedistribution of the aberration correcting optical system is approximatedby a function

Φ(r)=a×r ⁴ +b×r ²,

[0021] where Φ(r) is a phase at a radius r, r is a radius, and a and bare phase distribution coefficients.

[0022] With this arrangement, it is possible to easily realize theforegoing arrangement in which a magnitude of a phase in the phasedistribution increases with increase in distance from a point where theaberration correcting optical system crosses an optical axis of lightemitted from the light source.

[0023] Further, the phase distribution of the foregoing arrangement canbe easily realized by adjusting a voltage distribution applied to theliquid crystal element, when the liquid crystal element, for example, isused as the aberration correcting optical element.

[0024] Therefore, it is possible to easily realize the optical pickupdevice of the present invention.

[0025] In order to attain the foregoing object, the optical pickupdevice of the present invention is arranged so that the phasedistribution coefficients a and b satisfy:

a×b>0; or

a×b<0 and {−b/(6×a)}^((1/2)) >R,

[0026] where R is an effective radius of the aberration correctingoptical system.

[0027] With this arrangement, since the phase distribution of theaberration correcting optical system contains no inflection point withinthe effective radius of the aberration correcting optical system, thesecond derivative Φ″(r) of the phase distribution, which indicates therate of change in the slope of the phase, can have a small value evenwhen the centers of the objective lens and the aberration correctingoptical system are misaligned. This ensures that aberration is reduced.In order to attain the foregoing object, the optical pickup device ofthe present invention may be arranged so that the phase distributioncoefficient a satisfies:

|12×a×R ²|<0.002.

[0028] With this arrangement, the aberration can be reduced to 0.03 λrmsor smaller in the optical system having an NA of 0.85, when the amountof shift of the objective lens is assumed to be approximately 0.3 mm.

[0029] In order to attain the foregoing object, the optical pickupdevice of the present invention is so arranged that the objective lenshas an NA of not less than 0.75; and the aberration correcting opticalsystem comprises a liquid crystal element.

[0030] With this arrangement, it is possible to carry out recording andreproducing at high density by sufficiently focusing a beam spot,because the objective lens has a high NA. Further, it is possible toeasily correct an aberration because the liquid crystal element is usedas the aberration correcting optical element.

[0031] Further, in order to attain the foregoing object, an opticalpickup device of the present invention which includes an aberrationcorrecting optical element for changing a wavefront of incident lightpassing through the aberration correcting optical element, the wavefrontbeing an equiphase surface, is so arranged that the aberrationcorrecting optical element includes an electrode formed on a substrate,an optical medium whose refractive index with respect to the incidentlight changes in accordance with a voltage applied to the electrode, anda driving circuit which applies a voltage to the electrode; the drivingcircuit applies a voltage to the electrode so as to change therefractive index of the optical medium and thereby shift a phase of thewavefront of the incident light passing through the optical medium; andthe driving circuit applies a voltage to the electrode so that an amountof phase shift caused by the optical medium monotonously increases ordecreases in accordance with a distance from an optical axis of theincident light.

[0032] In the optical pickup device of the foregoing arrangement,incident light from, for example, the light source falls on theobjective lens via the aberration correcting optical element. The lightpasses through the objective lens and is condensed on a recordingsurface of an optical disk, for example. The reflected light from theoptical disk may be read to reproduce information recorded on theoptical disk. Further, the condensed light may be used to recordinformation on the recording surface. With the foregoing arrangement, anaberration of the light condensed on the recording surface via theobjective lens can be eliminated by suitably changing the wavefront ofincident light, using the aberration correcting optical element, forexample.

[0033] With this arrangement, even if the center of the objective lensis not aligned with the center of the aberration correcting opticalsystem, the rate of change in the slope of the phase is small because anamount of phase shift monotonously changes. Therefore, the centermisalignment does not cause a large aberration if the objective lens andthe aberration correcting optical system have been appropriatelyadjusted in a state having no center misalignment.

[0034] In order to attain the foregoing object, the optical pickupdevice of the present invention is so arranged that the driving circuitapplies a voltage to the electrode so as to form a phase distribution asa function of radius r when R>r>0:

Φ(r)=a×r ⁴ +b×r ²,

[0035] where the radius r is the distance from the optical axis of theincident light, and R is an effective radius of the aberrationcorrecting optical system.

[0036] With this arrangement, it is possible to realize theabove-described arrangement in which an amount of phase shift caused bythe optical medium monotonously increases or decreases in accordancewith a distance from an optical axis of the incident light.

[0037] In order to attain the foregoing object, the optical pickupdevice of the present invention is so arranged that the driving circuitapplies a voltage to (A) a center electrode which is located on theoptical axis of the incident light and (B) a circular electrode which islocated around the center electrode, so that an amount of phase shiftcaused by the optical medium monotonously increases or decreases inaccordance with the distance from the optical axis of the incidentlight.

[0038] With this arrangement, it is possible to easily realize theforegoing arrangement by applying a voltage to the central electrode andone or more circular electrodes so that the electrodes respectively havepredetermined voltages.

[0039] For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0040]FIG. 1 is a block diagram showing an embodiment of an opticalpickup device of the present invention.

[0041]FIG. 2 is a cross-sectional view of an optical recording medium ofFIG. 1.

[0042]FIG. 3 is a block diagram of an aberration correcting opticalsystem of FIG. 1.

[0043]FIG. 4(a) is a plan view showing how electrodes are arranged inthe liquid crystal element of FIG. 3;

[0044]FIG. 4(b) is a cross-sectional view showing how the electrodes arearranged in the liquid crystal element of FIG. 3; and

[0045]FIG. 4(c) is a graph showing a relationship between position andelectric field intensity in the liquid crystal element.

[0046]FIG. 5 is a plan view showing another example how electrodes arearranged in the liquid crystal element.

[0047]FIG. 6 is a graph showing a phase distribution of the aberrationcorrecting optical system of FIG. 1.

[0048]FIG. 7 is a graph showing a change in the slope of the phasedistribution of the aberration correcting optical system of FIG. 1.

[0049]FIG. 8 is a graph showing a phase distribution of a conventionalaberration correcting optical system.

[0050]FIG. 9 is a graph showing a change in the slope of the phasedistribution of the aberration correcting optical system of theconventional example.

BEST MODE FOR CARRYING OUT THE INVENTION

[0051] The following will explain an embodiment of the present inventionwith reference to attached drawings.

[0052] As shown in FIG. 1, an optical pickup device of the presentembodiment is so arranged that linearly polarized laser beam emittedfrom an LD (Laser Diode) 1 is converted into a parallel ray at acollimator lens 2 and enters a shaping prism 3. The shaping prism 3shapes an elliptical intensity distribution of the laser beam emittedfrom the LD 1 so that the intensity distribution has an approximatelycircular shape.

[0053] After this, the light exiting from the shaping prism 3 passesthrough a polarizing bean splitter 4, and enters an aberrationcorrecting optical system 5. Then, the light is converted intocircularly polarized light at a quarter-wave plate 6. The ray of lightthat is converted into circularly polarized light by the quarter-waveplate 6 is deflected by a 45-degree mirror (not shown), and an objectivelens 7 focuses the light onto an optical recording medium 10.

[0054] Note that, the optical recording medium 10 is composed of a lighttransmission layer 8 having a thickness of approximately 0.1 mm (medianand mean of the thickness are 0.1 mm), a recording surface 9, and asubstrate 11, as shown in FIG. 2. Referring to FIG. 1, the objectivelens 7 focuses light on the optical recording medium 10. Morespecifically, the light focused by the objective lens 7 (FIG. 1) passesthrough the light transmission layer 8, forms a beam spot on therecording surface 9, and follows the path as described below after beingreflected by the recording surface 9.

[0055] Namely, the light reflected by the recording surface 9 (FIG. 2)is restored to linearly polarized light by the quarter-wave plate 6, asshown in FIG. 1. Note that, the light is restored to the linearlypolarized light so that (i) the polarization direction of the light thatenters the quarter-wave plate 6 through the polarizing beam splitter 4and (ii) the polarization direction of the light that is reflected bythe optical recording medium 10 and enters the polarizing beam splitter4 via the quarter-wave plate 6 are rotated by 90 degrees from eachother.

[0056] The light that is restored to the linearly polarized light by thequarter-wave plate 6 is bent at a substantially right angle by thepolarizing bean splitter 4. Then, the light passes through a condensinglens 12 and enters a light-receiving section 13.

[0057] Note that, the objective lens 7 is anchored on a lens holder (notshown). Further, the lens holder is fixed to an optical pickup devicemain unit (not shown) with four wires (not shown). Further, theobjective lens 7 of the present embodiment has an NA of 0.85. Theobjective lens 7 is designed so that it has almost no aberration when aparallel ray enters the objective lens 7 (so-called infiniteconjugation) and when the light transmission layer 8 has a thickness of0.1 mm. Note that, in the present embodiment, the laser beam emittedfrom the LD 1 has a wavelength of 405 nm, the diameter of an effectiveray is Φ3, and the focal length is 1.76 mm.

[0058] An aberration detecting circuit calculates an amount of sphericalaberration based on an optical signal detected by the light-receivingsection 13. A liquid crystal driving circuit drives a liquid crystalelement of the aberration correcting optical system 5 in accordance witha signal regarding the calculated amount of spherical aberration. Theliquid crystal element will be described later. Note that, the sphericalaberration may be detected by either observing an amplitude and envelopeof an RF signal or providing a light-receiving section that can detectspherical aberration.

[0059] As shown in FIG. 3, the aberration correcting optical system 5 isa liquid crystal element 14 made of a nematic liquid crystal compositioncommonly used for liquid crystal displays and the like. Morespecifically, the liquid crystal element 14 is arranged so that liquidcrystal 15 is sandwiched between a pair of transparent electrodes whichface each other. The transparent electrode is prepared by, for example,depositing an ITO film on a glass substrate. In this arrangement, thealignment direction of liquid crystal molecules in the liquid crystal 15can be changed from horizontal to vertical by adjusting a voltageapplied across the transparent electrodes. Further, the liquid crystal15 has birefringence, in which a refractive index in a direction of theoptical axis and a refractive index in a direction perpendicular to theoptical axis are different from each other in the liquid crystalmolecules. With this, the incident light on one side of the liquidcrystal element 14 passes through the liquid crystal and is subjected tobirefringent effect therein in accordance with the alignment state ofthe liquid crystal 15. The light then emerges from the other side of theliquid crystal element 14. Therefore, the light that enters the liquidcrystal element 14 becomes linearly polarized light whose polarizationdirection is determined by the alignment method of the liquid crystal15.

[0060] The liquid crystal element 14 includes a first glass plate 16,the liquid crystal 15, and a second glass plate 17. The liquid crystal15 is sandwiched between the first glass plate 16 and the second glassplate 17 which face each other.

[0061] A transparent electrode (electrode) 18, an insulating layer 19,and an alignment layer 20 are formed on the first glass plate 16. On theother hand, a transparent electrode (electrode) 21, an insulating layer22, and an alignment layer 23 are formed on the second glass plate 17.The electrode 21 is a common electrode having a circular shape. Theliquid crystal 15 is sealed with a sealing material 27.

[0062] Further, as shown in FIG. 4(a), the transparent electrode 18 onthe first glass plate 16 includes electrodes 18 a, 18 b, and 18 c whichare respectively formed in separate concentric areas. The transparentelectrode 18 further includes metal electrodes 18 d, one of which isformed on the center of the electrode 18 c, and the others arerespectively formed on border lines of the divided areas. Further, themetal electrodes 18 d are respectively connected with lead wires 26 a,26 b, and 26 c. Note that, the transparent electrode 21 on the secondglass plate 17 is a common electrode having a circular shape.

[0063] As shown in FIG. 4(b), the transparent electrode 18 is arrangedby combining the transparent electrodes (such as ITO) 18 a through 18 chaving high resistance and the metal electrodes (such as gold andaluminum) 18 d having low resistance. With this arrangement, thetransparent electrode 18 can form the electric field distribution inwhich the electric field is largest at the central portion and becomessmaller toward the peripheral portions as shown in FIG. 4(c). Thisgenerates a phase difference in the liquid crystal 15 (FIG. 3), therebyeliminating the aberration.

[0064] More specifically, by applying voltages V1=4V, V2=2V, and V3=1Vrespectively to the lead wires 26 a, 26 b, and 26 c for example, thetransparent electrode 18 can form the electric field distribution asshown in FIG. 4(c) such that the field intensity is largest at thecentral portion of the transparent electrode 18 and becomes graduallysmaller toward the peripheral portions of the transparent electrodes 18.This enables the aberration correcting optical system 5 to impart asmall refractive index at the central portion, and a gradually largerrefractive index toward the peripheral portions.

[0065] Namely, by respectively controlling voltage values for theelectrodes 18 a, 18 b, and 18 c formed on the first glass substrate, itis possible to form a voltage distribution and thereby a refractiveindex distribution in accordance with the voltage distribution. This isbecause the refractive index of the liquid crystal changes as thealignment direction of the liquid crystal changes in response to theapplied voltages. As to the phase distribution corresponding to therefractive index distribution in the liquid crystal element 14 of thepresent embodiment, description will be given later.

[0066] The following will explain another example of an arrangement ofthe electrodes provided for the liquid crystal element 14 so as tochange the alignment state of the liquid crystal 15. Namely, theelectrodes may be so arranged that a terminal 25 for voltage applicationis provided at a central portion of a circular electrode 24, and a leadwire 26 for supplying power is provided so as to extend outward from theterminal 25, as shown in FIG. 5. This arrangement can form a voltagedistribution in which the voltage is the largest at the central portionand becomes smaller toward the peripheral portions. This enables theaberration correcting optical system 5 to impart a small refractiveindex at the central portion and a gradually larger refractive indextoward the peripheral portions.

[0067] Note that, the combination of the shapes and resistance values ofthe electrodes as described above is only an example. The shapes andresistance values of the electrodes may be suitably designed fordifferent types of pickup devices having different aberrations to becorrected.

[0068] Further, in the foregoing exemplary arrangement of theelectrodes, the electrodes having a high resistance and a low resistanceare combined to form the electric field with the largest central portionand smaller peripheral portions. Alternatively, the thickness of thetransparent electrode may be changed to suitably form a voltagedistribution.

[0069] Next, the following will explain a method to minimize an amountof spherical aberration in case where the light transmission layer 8 hasan uneven thickness in the optical recording medium 10.

[0070] For example, when the mean value (median value) of the thicknessof the light transmission layer 8 is 0.1 mm, the optical pickup deviceis arranged to enable recording and reproducing for a thickness of 0.1mm±0.015 mm. In other words, to accommodate a multi-layer recordingmedium, the optical pickup device is arranged to enable recording andreproducing as long as the thickness of the light transmission layer 8falls within a predetermined allowable range, even if the thickness doesnot exactly have a predetermined mean value. The spherical aberrationcaused by the uneven thickness of the light transmission layer 8, andthe corresponding spherical aberration on the recording layer 9 can becorrected by varying the voltages applied to the electrodes 18 a, 18 b,and 18 c of the liquid crystal element 14.

[0071] Next, description is made as to how spherical aberration iscorrected by the phase distribution in the liquid crystal element 14.

[0072]FIG. 6 shows a phase distribution of the liquid crystal element 14when the thickness of the light transmission layer 8 is 0.115 mm (a gainof +15 μm).

[0073] Using a position r distanced in a radius direction from thecenter of the liquid crystal element 14 provided as an aberrationcorrecting element, the phase distribution shown in FIG. 6 isapproximated by a polynominal expression as follows.

Φ(r)=0.000021083×r ⁴−0.001033×r ².

[0074] Further, a second-order differential function Φ″(r), whichindicates the rate of change in the slope of the phase distributionΦ(r), is expressed as follows.

Φ(r)=0.000252996×r ²−0.002066.

[0075] Note that, FIG. 7 shows the rate of change in the slope of thephase distribution Φ(r) of the present invention. FIG. 7 also shows therate of change in the slope of a conventional phase distribution.

[0076] As is clear from FIG. 7, Φ″(r) of the present embodiment has onlynegative values, so that the phase distribution Φ(r) has no inflectionpoint.

[0077] In the conventional arrangement as shown in FIG. 8 for example,the phase distribution has an inflection point because there are pointsat which the rate of change of the slope is 0 as shown in FIG. 7. Inthis case, the phase distribution has a minimum value at positions inthe vicinity of, for example, r=±1.1 mm, not the center r=0.

[0078] The light is subjected to the phase distribution as it passesthrough the liquid crystal element, and enters the objective lens afterits wavefront as an equiphase surface is transformed according to thephase distribution in the liquid crystal element.

[0079] Therefore, when the centers of the objective lens and the liquidcrystal element are aligned with each other, the wavefront of incidentlight on the objective lens has no disturbance on positionscorresponding to the minimum value of the phase distribution. On theother hand, when the centers of the objective lens and the liquidcrystal element are misaligned with each other, the wavefront ofincident light on this part of the objective lens has a disturbanceaccording to a direction of the misalignment. Consequently, a wavefrontof the opposite phase may be incident depending on the direction of themisalignment. This may cause a large aberration in this part of theobjective lens, for example.

[0080] In contrast, in the arrangement shown in FIG. 7, the phasedistribution has no inflection point. Accordingly, the phasedistribution does not have a minimum value at a point other than thecenter of the phase distribution. With this, a large aberration does notoccur because the wavefront is not seriously disturbed at points otherthan the center of the phase distribution. As a result, wavefrontaberration can be reduced.

[0081] Further, as shown in FIG. 7, a second derivative value of thephase distribution of the present embodiment is sufficiently small atpositions with a radius of 1.5 mm, i.e., the outermost portion of theeffective diameter of the laser beam. With this, the wavefront of theincident light on a particular portion of the objective lens is notgreatly disturbed even when the centers of the objective lens and theliquid crystal element are misaligned. As a result, wavefront aberrationcan be reduced.

[0082] Here, at positions in the vicinity of R=±0.6 mm, the phasedistribution of the conventional arrangement has points at which therate of change in the slope of the phase distribution changes its sign.For example, when the objective lens is shifted in a positive direction(direction of increasing radius) with respect to the liquid crystalelement, the rate of change in the slope of the phase distributiondecreases at r=+0.6 mm. On the other hand, with this shift, the rate ofchange in the slope of the phase distribution at a position of r=−0.6 mmincreases and has a sign opposite to that at r=+0.6 mm. This causes alarge aberration.

[0083] Incidentally, the phase distribution imparted by the liquidcrystal element 14 in accordance with an amount of spherical aberrationis generally expressed as follows.

Φ(r)=a×r ⁴ +b×r ²

[0084] Accordingly, a second-order differential function Φ″(r) of Φ(r)is expressed as follows.

Φ″(r)=12a×r ²+2×b

[0085] It is assumed that R is an effective radius of the aberrationcorrecting optical system. Here, it is required that Φ″(r)≠0 within arange of R>r>0 in order to meet the condition that Φ(r) has noinflection point within the range of R>r>0. Namely, it is necessary tosatisfy the following:

[0086] a>0 and b>0; or

[0087] a<0 and b<0; or

[0088] {−b/(6×a)}^((1/2))>R.

[0089] From this it follows that Φ(r) has no inflection point within therange of R>r>0 when:

[0090] a×b>0; or

[0091] {−b/(6×a)}^((1/2))>R are satisfied where a×b<0.

[0092] Further, within the range of R>r>0, the objective lens has thelargest incident angle in the vicinity of the effective radius R. Thus,when the wavefront of incident light has a large disturbance (namelyincident angle) in the vicinity of the effective radius R, moreaberration is caused. This means that more aberration is caused when thecenters of the objective lens and the liquid crystal element aremisaligned.

[0093] Therefore, it is necessary that the value of Φ″(r) issufficiently small in the vicinity of the effective radius (outermostportion) of the aberration correcting optical system, in order to reduceaberration.

[0094] Here, it is necessary to provide a margin of approximately 0.3 mmfor the shift of the objective lens. Thus, in order to obtain anaberration of not more than 0.03 λrms, the optical system having an NAof 0.85 is required to satisfy the following relationship.

|12×a×R ²|<0.002

[0095] When the phase distribution of the liquid crystal elementcorrecting a predetermined spherical aberration satisfies the foregoingcondition, the wavefront emerging from the liquid crystal element andentering the objective lens will not be seriously disturbed even whenthe centers of the liquid crystal element and the objective lens aremisaligned. As a result, aberration can be reduced.

[0096] Next, Table 1 shows aberration values that resulted from shiftingof the objective lens in tracking in a direction of the track widthwhen, for example, the optical pickup device of the present embodimentwas used to correct aberration for a light transmission layer having athickness of 115 μm (for reference, Table 1 also shows aberration valuesthat resulted from the phase distribution of the conventional example).Note that, mλ (millilambda), which is shown as the unit of aberrationvalue, is a rms (root mean square) value, but the notation rms isomitted in the table. TABLE 1 CENTER MISALIGNMENT 0 mm 0.1 mm 0.2 mmPRESENT 12 mλ 13 mλ  15 mλ EMBODIMENT CONVENTIONAL 12 mλ 72 mλ 151 mλEXAMPLE

[0097] By comparing the present embodiment and the conventional example,it can be seen from Table 1 that the aberration values in the presentembodiment are sufficiently smaller than the aberration values in theconventional example for a given amount of center misalignment.

[0098] Note that, even though the present embodiment uses a liquidcrystal element as the aberration correcting element, other elements mayalso be used as long as the phase distribution as described above can beobtained. For example, the effects of the present embodiment can also beobtained with other elements, including one using a material whoserefractive index can be adjusted by changing the applied voltage.Further, the spherical aberration values obtained in this example arebased on the light transmission layer with a thickness range of ±15 μm,but the thickness of the light transmission layer is not limited to thisand may be determined by a spherical aberration to be corrected indifferent systems.

[0099] Further, the objective lens having an NA of 0.85 is used in thepresent embodiment. Generally, a pickup which uses an objective lenswhose NA is not less than 0.75 causes a large amount of sphericalaberration with respect to a change in thickness of the lighttransmission layer in two-layer recording and the like. Therefore, withthe liquid crystal element (aberration correcting element) of thepresent embodiment, strong aberration correction effects can beobtained.

[0100] Incidentally, a tertiary spherical aberration coefficient W40 isexpressed as follows.

W40≈(t/8)×{(n ²−1)/n ³}×(NA)⁴

[0101] Namely, for a given change in the thickness of the lighttransmission layer, the amount of spherical aberration for NA=0.75 istwo times or greater than that for NA=0.6. In a two-layer recording(reproducing) medium, a thickness between the layers is determined bysuch factors as thermal interference between the recording layers,interference of a focus servo signal, and a method for manufacturing aninterlayer layer Specifically, the thickness between the layers is atleast about 10 μm to 20 μm. Due to unevenness in the thickness betweenthe layers, the amount of spherical aberration exceeds the allowableaberration value of 0.03 λrms when NA is 0.75.

[0102] Therefore, in an optical system having such a high NA, theaberration can be reduced more effectively and characteristics of theoptical pickup can be improved by using the aberration correctingelement of the present embodiment. The aberration correcting element ofthe present invention is particularly important in an optical systemhaving an objective lens whose NA is not less than 0.75, because theaberration needs to be suppressed at low level in such an opticalsystem.

[0103] In one aspect of the invention, the present invention provides anoptical pickup device including an aberration correcting optical elementfor changing a wavefront of incident light passing through theaberration correcting optical element, the wavefront being an equiphasesurface, in which the aberration correcting optical element includes anelectrode formed on a substrate, an optical medium whose refractiveindex with respect to the incident light changes in accordance with avoltage applied to the electrode, and a driving circuit which applies avoltage to the electrode; the driving circuit applies a voltage to theelectrode so as to change the refractive index of the optical medium andthereby shift a phase of the wavefront of the incident light passingthrough the optical medium; and the driving circuit applies a voltage tothe electrode so that an amount of phase shift caused by the opticalmedium monotonously increases or decreases in accordance with a distancefrom an optical axis of the incident light.

[0104] The optical pickup device of the present invention may be soarranged that the driving circuit applies a voltage to the electrode soas to form a phase distribution as a function of radius r when R>r>0:

Φ(r)=a×r ⁴ +b×r ²,

[0105] where the radius r is the distance from the optical axis of theincident light, and R is an effective radius of the aberrationcorrecting optical system.

[0106] Further, the optical pickup device of the present invention maybe so arranged that the driving circuit applies a voltage to (A) acenter electrode which is located on the optical axis of the incidentlight and (B) a circular electrode which is located around the centerelectrode, so that an amount of phase shift caused by the optical mediummonotonously increases or decreases in accordance with the distance fromthe optical axis of the incident light.

[0107] As described above, the optical pickup device of the presentinvention has a large tolerance for the center misalignment with theobjective lens, thereby eliminating the need for mounting the aberrationcorrecting element on the actuator together with the objective lens.This reduces the weight of the actuator, thereby realizing high-speeddriving of the actuator.

[0108] Further, it is possible to stably record and reproduceinformation on and from an optical disk by providing the optical pickupdevice of the present invention in an optical disk reproducing device oran optical disk recording-reproducing device.

[0109] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

INDUSTRIAL APPLICABILITY

[0110] An optical pickup device of the present invention can preventdeterioration due to center misalignment of an objective lens with aliquid crystal element even when the objective lens has a high NA. Thus,the optical pickup device is suitable for increasing the recordingcapacity of an optical recording medium. Further, the optical pickupdevice can drive an actuator at high speed, by reducing the weight ofthe actuator.

1. An optical pickup device comprising: a light source; and an objectivelens and an aberration correcting optical system which are located in alight path from said light source to an optical recording medium,wherein: said aberration correcting optical system has a phasedistribution for a light beam passing through said aberration correctingoptical system, so as to correct a predetermined aberration; and saidaberration correcting optical system for correcting an aberration is setso that a magnitude of a phase in the phase distribution increases withincrease in distance from a point where said aberration correctingoptical system crosses an optical axis of light emitted from said lightsource.
 2. The optical pickup device as set forth in claim 1, wherein:the phase distribution of said aberration correcting optical system isapproximated by a function Φ(r)=a×r ⁴ +b×r ², where Φ(r) is a phase at aradius r, r is a radius, and a and b are phase distributioncoefficients.
 3. The optical pickup device as set forth in claim 2,wherein: the phase distribution coefficients a and b satisfy: a×b>0; ora×b<0 and {−b/(6×a)}^((1/2))>R, where R is an effective radius of saidaberration correcting optical system.
 4. The optical pickup device asset forth in claim 3, wherein: the phase distribution coefficient asatisfies: |12×a×R ²|<0.002.
 5. The optical pickup device as set forthin any one of claims 1 through 4, wherein: said objective lens has an NAof not less than 0.75; and said aberration correcting optical systemcomprises a liquid crystal element.
 6. An optical pickup devicecomprising an aberration correcting optical element for changing awavefront of incident light passing through the aberration correctingoptical element, the wavefront being an equiphase surface, wherein: saidaberration correcting optical element includes an electrode formed on asubstrate, an optical medium whose refractive index with respect to theincident light changes in accordance with a voltage applied to saidelectrode, and a driving circuit which applies a voltage to saidelectrode; said driving circuit applies a voltage to said electrode soas to change the refractive index of said optical medium and therebyshift a phase of the wavefront of the incident light passing through theoptical medium; and said driving circuit applies a voltage to saidelectrode so that an amount of phase shift caused by said optical mediummonotonously increases or decreases in accordance with a distance froman optical axis of the incident light.
 7. The optical pickup device asset forth in claim 6, wherein: said driving circuit applies a voltage tosaid electrode so as to form a phase distribution as a function ofradius r when R>r>0: Φ(r)=a×r ⁴ +b×r ², where the radius r is thedistance from the optical axis of the incident light, and R is aneffective radius of said aberration correcting optical system.
 8. Theoptical pickup device as set forth in claim 6, wherein: said drivingcircuit applies a voltage to (A) a center electrode which is located onthe optical axis of the incident light and (B) a circular electrodewhich is located around the center electrode, so that an amount of phaseshift caused by said optical medium monotonously increases or decreasesin accordance with the distance from the optical axis of the incidentlight.